1. Field of the Invention
The invention relates to a method and apparatus for cementing a casing string within a well bore.
2. Description of the Prior Art
When a well for the production of hydrocarbons is drilled, it is necessary to place lengths, or strings, of pipe, or casing, in the well bore to various depths to permit the well driller to control the well while drilling operations are ongoing, as well as to allow control of the well while the hydrocarbons, or oil or gas, are being produced. These strings of pipe, or a casing string, are typically permanently installed in the well bore by cementing them to the well bore. In some instances a casing string is permanently installed, as by cementing, within a larger diameter casing string, or cased hole. As used herein, the term "well bore" means either an open, or drilled, bore hole, or a cased hole, which already has a length of casing installed therein. Typically, the strings of pipe, or casing string, are permanently installed within the well bore by pumping cement into the annular cavity between the outside of the pipe, or casing, and the well bore.
In some instances due to well conditions, the entire length of casing string cannot be cemented within the well bore along the entire length of the casing string by pumping cement outwardly from the lower end of the casing string and then upwardly along the entire length of the casing string. In these instances, a down-hole tool, such as a stage cementing collar is disposed in the casing string between its upper and lower ends, generally somewhere near the middle of the casing string. The tool is operable to permit cement to be pumped outwardly through the tool and into the annular cavity between the casing string and the well bore. Typically the tool is operated to temporarily open fluid ports in the tool, through which the cement may pass into the annular cavity. The foregoing procedure is generally known in the art as "two-stage cementing", wherein cement is mixed and pumped into the annular cavity between the casing string and the well bore from two different locations along the length of the casing string, the first location being the bottom of the casing string, and the second location being adjacent the tool having the fluid ports. There are presently a variety of tools and methods in use to accomplish two-stage cementing.
One method and apparatus for two-stage cementing involves dropping a weighted device down the casing string which lands in a sleeve in a cementing collar and seals the inner bore of the cementing collar, whereby fluid cannot pass down the casing string beyond the weighted device. Fluid pressure is applied to the cementing collar, as by pumping cement into the casing string and cementing collar. This pressure force causes the shearing of shear pins which releasably secure an opening sleeve over fluid ports extending from the interior of the cementing collar to the annular cavity between the casing string and the well bore. After the desired amount of cement has been pumped through the cementing collar, a rubber closing plug is placed in the casing string and pumped to the cementing collar until the rubber closing plug lands on another sleeve, or closing sleeve, within the cementing collar. The rubber closing plug is also disposed in a sealing relationship within the cementing collar, whereby upon the pumping of a fluid, such as drilling mud, into the casing string and cementing collar, the rubber closing plug applies a force to the closing sleeve. This force is sufficient to shear a plurality of shear pins which initially hold the closing sleeve in an open, fluid transmitting relationship with the exterior of the cementing collar. Upon the shear pins of the closing sleeve being sheared, the closing sleeve moves downwardly to seal off the fluid ports of the cementing collar.
In another method and apparatus for two-stage cementing, a cementing collar, disposed within the casing string, is opened by the application of fluid pressure acting upon an opening sleeve of the cementing collar. Shear pins, or shear balls, initially restrain the desired downward movement of the opening sleeve, until the necessary hydraulic force has been applied to the opening sleeve to shear the pins or balls. After a desired quantity of cement is pumped through the exposed fluid ports, a rubber closing plug is pumped downwardly through the casing string until it lands on another sleeve, or closing sleeve, and seats therein in a sealing relationship. By continuing to apply fluid pressure to the closing plug, shear pins, or shear balls, which initially restrain the movement of the closing sleeve, are sheared and the closing sleeve moves downwardly to seal the fluid ports in the cementing collar.
A further two-stage cementing method and apparatus utilizes a cementing tool having a plurality of fluid ports disposed within the casing string. This cementing tool has its fluid ports opened and closed by another tool which is run into the casing string on a smaller diameter string of pipe, such as production tubing. When the second tool is disposed within the cementing tool, it may operated to open or close the set of ports located in the cementing tool, typically either by rotating a sleeve within the cementing tool, or by moving a sleeve within the cementing tool either upwardly or downwardly to open and close the fluid ports.
Each of the foregoing two-stage cementing methods and apparatus have certain disadvantages associated with them, particularly when the well bore is a deviated hole, wherein the lower end of the casing string is disposed at an angle, with respect to the upper end of the casing string, which can approach 90.degree. in the case of certain horizontally drilled wells. For example, in methods and apparatus requiring the dropping of a weighted device into the cementing collar, some times such weighted opening devices do not reach the cementing collar to perform their desired function of opening the cementing collar, because of resistance encountered from the viscous fluid, such as drilling mud, disposed in the casing string. Particularly in deviated well bores, the gravity force acting upon the weighted opening device may not be sufficient to overcome the frictional drag force of the opening device upon the interior surface of the casing string at locations where the casing string is not perpendicular to the earth's surface.
Another disadvantage associated with methods and apparatus which require weighted opening devices and/or rubber closing plugs is that after the desired cementing operation has been completed, it is then necessary to expend valuable drilling rig time to drill out the rubber closing plug and/or weighted opening device, in order to clear the interior bore of the casing string. With respect to two-stage cementing methods and apparatus which require the use of another tool lowered through the casing string to the cementing collar, particularly when the cementing collar sleeve is opened by upward or downward movement of the opening/closing tool, every time the opening/closing tool passes through the cementing collar, the fluid ports will either be automatically opened or closed. In such equipment, it is not possible to pass the opening/closing tool through the cementing collar without opening or closing the fluid ports. Thus, it is not possible with such equipment to use the opening/closing tool to perform any other functions, or operations, such as inflating a packer disposed below the cementing collar, because once the opening/closing tool passes the cementing collar, it may automatically effect an undesired opening or closing of the fluid ports. Accordingly, another separate downhole tool must be lowered through the casing string and below the cementing collar to perform other tasks in the casing string, which tool must then be pulled out of the well bore, at which time the opening/closing tool must be attached to the smaller diameter tubing and lowered into the casing string to open or close the cementing collar. In the prior art two-stage cementing apparatus and methods which require rotation of an opening/closing tool suspended from a tubing string, it is very difficult to operate such equipment to open or close a cementing collar in a deviated well bore. Additionally, increased rig time is involved for lowering the opening/closing tool to the cementing collar.
With respect to all the foregoing prior art two-stage cementing methods and apparatus, another disadvantage associated with such equipment is that the construction of such equipment is relatively complex and expensive, and require a multitude of separate components to be assembled in order for the equipment to be operable. Another disadvantage of some prior art two-stage cementing equipment is that they utilize seals which may be in the flow-path of the cement which passes through the fluid ports, whereby a possibility exists that such seals may be eroded, or cut, by the cement flowing out of the flow port. Such erosion, or cutting, can result in well fluids leaking out of the tool which is undersireable.
Accordingly, prior to the development of the present invention, there have been no methods and apparatus for cementing a casing string which: are simple, economical, and efficient to manufacture and use; do not require weighted opening devices and/or rubber closing plugs; permit an opening/closing tool to pass through the cementing collar without always either opening or closing the fluid ports of the cementing collar; permit an opening/closing tool to perform other desired functions within the casing string disposed below the cementing collar; and utilize seals for the fluid ports which are not in the flow path of the cement passing through the fluid ports.
Therefore, the art has sought methods and apparatus for cementing a casing string in a well bore which: are simple, economical and efficient to manufacture and use; do not require the use of weighted opening devices and/or rubber closing plugs; permit an opening/closing tool to pass through the cementing tool without always either opening or closing the cementing tools; permit an opening/closing tool to also be used to perform additional functions and tasks in the casing string below the cementing tool; and utilize seals which are not in the flow-path of the cement which passes through the fluid ports.